A conductive polymer has such a high conductivity that it can be used as a solid electrolyte of a solid electrolytic capacitor such as tantalum solid electrolytic capacitor, aluminum solid electrolytic capacitor and niobium solid electrolytic capacitor.
As a conductive polymer in such an application, ones obtained from thiophene or its derivatives can be used, which are prepared through chemical oxidative polymerization or electrolytic oxidation polymerization.
As a dopant in performing the chemical oxidative polymerization of the thiophene or its derivatives, organic sulfonic acids can be mainly used. Among them, it is said that an aromatic sulfonic acid is preferred. As an oxidant, transition metals can be used, and among them, a ferric compound is said to be preferred. Usually, an aromatic sulfonic acid ferric salt can be used to serve as an oxidant and dopant agent to perform the chemical oxidative polymerization of thiophene or its derivatives.
Regarding thiophene or its derivatives, considering the balanced conductivity and heat resistance of the obtained conductive polymer as well as its utilization, 3,4-ethylenedioxythiophene has been often used. See Patent Publications Nos. 1 and 2 below.
However, the innovation of the solid electrolytic capacitor using a conductive polymer as a solid electrolyte is rapid. There is, therefore, demanded to further improve the properties.
Then, in order to improve the conductivity, there was a proposal to use 3,4-alkylenedioxy thiophene in which 3,4-ethylenedioxythiophene has a substitute of alkyl group. See Patent Publication No. 3 listed below. However, when using 3,4-alkylenedioxy thiophene, the heat resistance is significantly decreased, and therefore, when using it as a solid electrolyte of a solid electrolytic capacitor, the reliability of the solid electrolytic capacitor under a hot temperature condition can be reduced. Also, the value of ESR (equivalent series resistance) can become large. It, therefore, results in enhancing the power consumption as well as deteriorating the noise reduction characteristics.